In general, this invention provides an improvement of quality and control of tone in an audio amplifier system. Various musical instruments, such as a guitar, may be used with electronic sound pickup devices mounted on the instrument for connection to an amplifier system. The quality of the sound generated through the amplifier system varies substantially due to variations in the quality of the instrument, the pickup device and the amplifier system.
U.S. patents, Nos. 4,523,156 and 4,539,526, the disclosures of which are incorporated herein by reference, disclose apparatus to prevent distortion due to clipping and to prevent reproduction of transient signals introduced by power or input switching. As disclosed in these patents, prior art circuits for controlling tone in an audio amplifier have used capacitors which appear as open circuits at low frequencies with a potentiometer defining circuit gain at low frequencies (bass). At approximately 1 kHz the capacitors are much lower in impedance than one potentiometer and higher in impedance than another potentiometer so that the capacitors set circuit gain to unity at this frequency. At higher frequencies (treble) the other potentiometer is much lower in impedance than both the one potentiometer and the capacitors, such that circuit gain is controlled by the other potentiometer.
In almost any audio system the volume level or the bass (tone) controls may be increased so that the output signal at a speaker becomes very distorted, usually as the result of signal clipping. It is usually difficult to provide the desired amount of control of amplifier dynamic range in both volume and tone without encountering this problem. It is possible to still allow for the desired head room and yet prevent clipping by detecting any clipping present in the audio signal at the power amplifier, and using the detected clipping signal to decrease the bass and volume levels.
In an audio reproduction system, it is far more likely that high peak signals that can produce clipping will be encountered in lower frequencies than at higher frequencies. Since the bass is very likely to contain such high level signals, it is desirable to decrease bass response before decreasing volume level. In this way, clipping due to excess energy in the bass region of the signal is prevented without effecting the overall dynamic range of the program material presented to the amplifier. If, after decreasing response at the bass frequencies the distortion still persists, then the overall volume level may be reduced to prevent such clipping or distortion. A further type of interference is encountered when switching transients are produced by power switching or input source switching. Such transients produce a disagreeable loud pip which is reproduced by the system speakers.
Typical tone control circuits employ an operation amplifier or other such amplifier having a differential input. The amplifier's output is fed back to the negative input of the amplifier through a low frequency (bass) feedback network controlled by a high resistance potentiometer and through a high frequency (treble) feedback network controlled by a second, lower resistance potentiometer. Two capacitors filter low frequency signals from the signal flowing through the second potentiometer and define the gain of the tone control circuit as unity for midrange frequencies.
U.S. Pat. No. 4,523,156 relates to initially reducing bass response and then overall signal level in an audio amplifier to prevent clipping and to attenuate power and switching transients. An electronic variable resistor is incorporated into a tone control circuit. Initial bass response control and, thereafter, volume level control are provided when clipping or transient interference is detected. An electronically variable resistor is coupled between the output of the amplifier and the amplifier's input. By varying the resistance in response to that sensed dynamic levels and transients, the circuit bass response is initially reduced. Thereafter, overall volume level is reduced to further prevent clipping and other such distortion, should bass response reduction fail to provide adequate compensation.
U.S. Pat. No. 4,539,526 discloses an adaptive signal weighting system for use in transmitting an electrical information signal of a predetermined bandwidth along a signal path. The system can be used to encode or decode the signal. The system comprises filter means disposed in the signal path for varying the gain impressed on the portion of the information signal within a first select spectral region within the preselected bandwidth. The gain is varied in response to and as a function of a first control signal. Means are provided for generating the first control signal in response to and in accordance with the signal energy of the information signal substantially within at least a part of the first select spectral region. The system also comprises gain control means disposed in the signal path and coupled to the filter means for varying the signal gain impressed on the information signal substantially throughout the predetermined bandwidth. The signal gain is variable in response to and as a function of a second control signal. A second control signal is generated in response to and as a function of the signal energy of the information signal within at least one other select spectral region of the information signal.
The present invention provides an audio signal conditioning system comprising power supply means for supplying DC voltage to the system; audio input signal receiving means for receiving audio input signals from an audio signal source; a first high gain transistor amplifier means having a transistor base collector and emitter for receiving the audio input signals at the base thereof and providing a first amplified output signal from the collector thereof; a second transistor amplifier means having a base, collector and emitter connected to said first transistor amplifier means for receiving said first amplified output signal and providing a second amplified output signal from the collector thereof; a first transistor connector circuit means for directly connecting the collector of the said first transistor amplifier means to the base of said second transistor amplifier means; a second transistor connector filter circuit means for connecting the collector of said first transistor amplifier means to the collector of said second transistor amplifier means for transmission of high frequencies of said first amplified output signal; a first high pass filter input circuit means with first variable resistor means for selectively transmitting the audio input signals to said first amplifier means; a second normal frequency amplification input circuit means with a second variable resistor means for selectively transmitting the audio input signals to said first amplifier means; a switch means for selectively connecting the audio input signals to the base of one of said first and second amplifier means through one of said first and second input circuit means; a regenerative feedback circuit means (C10, C11, R12, C8) for connecting the collector of said second transistor means to the base of said first transistor means; an adjustable tone control circuit means in said regenerative feedback circuit means for selectively adjusting resistance therein; negative feedback circuit means for connecting the emitter of said second transistor means to the base of said first transistor means and filtering the bass response; an emitter bypass filter circuit means connected to the emitter of said second transistor means for increasing audio gain therein; an output circuit means connected to the collector of said second transistor means for outputting said second amplified audio signals therefrom; and output gain control means in said output circuit means for providing selectively variable resistance therein.